Let's take a step back. The purpose of a Save the Date card is to inform your guests of your engagement and allow them time to make advance travel plans, etc. Now, to answer the question regarding the etiquette, Save the Date cards are informal. They can be creative, they can tie into your theme and colors or they can be an individual expression. Rule #1 if you send them a Save the Date card you HAVE to send them an invitation. If you stick to those ideas and rules you will be just fine!
Your Save-the-Date cards should be mailed shortly after your engagement is announced. This will allow guests to reserve your date and for those guests that are out of town, to start making travel arrangements.
Save-the-Dates are typically more informal than wedding invitations, so have fun with them.
Save the dates can definitely be as unique or as fun as you'd like them to be. They are typically sent approximately 4 to 6 months ahead of the wedding date, especially if you have out of town guests. This way they can make travel arrangements and other plans surrounding the date of your wedding.
Here is a fun, party themed save the date we did for a bride having a Lollapalooza type of wedding. The cards should reflect the couple's personality and/or theme of the event. We do custom work for instances where bride's have a unique idea or perhaps want something one of a kind, and would love to share ideas with you if you need help.